This invention relates to the polymerization of monomers in a liquid diluent.
Addition polymerizations are frequently carried out in a liquid which is a solvent for the resulting polymer. When high density (linear) ethylene polymers first became commercially available in the 1950's this was the method used. It was soon discovered that a more efficient way to produce such polymers was to carry out the polymerization under slurry conditions. More specifically, the polymerization technique of choice became continuous slurry polymerization in a pipe loop reactor. Subsequent to the polymerization, the polymer must be separated from the diluent and the diluent recovered for recycle.
Preferably the solid catalyst for such reactions is first combined with diluent and the resulting catalyst mud introduced into the reactor.
Both environmental sensitivity and economics dictate that as little waste as possible occurs in such processes. Accordingly, modern slurry plants "lose" so little diluent during the separation and diluent recovery process that only a minute amount of "make-up" diluent is required. Indeed, so little make-up diluent is needed that the make-up diluent is of insufficient quantity to produce the catalyst mud. Hence recycle diluent must be used. However, recycle diluent unavoidably contains some unreacted monomer dissolved in it. While some monomer can be tolerated, it is greatly preferred to have monomer-free diluent for this step. Further complicating the polymer separation and diluent recovery is the presence of small amounts of other undesirable compounds in the recycle diluent such as alkanes other than the diluent, olefins other than the monomer or monomers, very low molecular weight oligomers produced in the reactor such as dimers and trimers, iert materials such as nitrogen and, finally, oxygen which is a catalyst poison and must be removed or at least essentially removed.